The reason is that most people aren't doing dungeons because they enjoy doing them. They've done them a million times and so want to get out of there as fast as possible. Most people are either in there just to get enough gear to do something else or to get their valor points.
I'm pretty impatient but I don't go as far as pulling for the tank if he's actually there. If there's a tank that dc's or keeps going afk then I will pull and let my pet tank.... I don't need a tank in a heroic dungeon so I don't see any reason to wait or keep the other 3 people in the group waiting when we can continue just as quickly without the tank being there.